The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image areas and the water or fountain solution is preferentially retained by the non-image areas of the printing plate. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and plastics. Commonly, the ink is transferred to an intermediate material called the blanket which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plates can be either negative-working or positive-working, and comprise one or more photosensitive layers on a suitable substrate, such as a metal or polymeric support. The photosensitive layer generally includes one or more photosensitive components that may be dispersed in a suitable binder. Alternatively, the photosensitive component can also be the binder material. Certain useful printing plates can be used either as positive-working or negative-working.
A negative-working printing plate generally has a light sensitive layer composed of a photosensitive component such as an unsaturated resin on a suitable substrate. Upon exposure to light, the exposed areas are hardened, leaving non-exposed areas removable during development. The exposed, hardened areas are therefore oleophilic and will accept ink while the non-exposed underlying areas of the substrate are hydrophilic.
A positive-working printing plate generally has a light sensitive layer composed of a photosensitive component such as an o-diazoquinone or diazonaphthoquinone compound. Upon exposure to light the photosensitive component is converted to the corresponding carboxylic acid. The use of an alkaline developer will remove only the exposed areas of the photosensitive layer, leaving the surface of the support. Since the surface of the support is hydrophilic, the uncovered non-image area attracts water and repels the oily ink. The image area remaining after development is oleophilic thereby repelling water and attracting the printing ink.
Various aqueous solutions are known for use as developers for both positive-working and negative-working printing plates. It is known to use a dispersion of a silicate to develop positive-working printing plates, as described, for example, in U.S. Pat. No. 4,259,434 (Yamasue et al). These solutions include alkali metal silicates, in which the ratio of SiO.sub.2 to M is from 0.5 to 0.75 ("M" being the alkali metal) and the SiO2 concentration is about 1-4%. However, many of such developers are overly active and attack or remove the unexposed image on the plates. The replenishment rate of such developers is critical because the operational range of the developers is very narrow.
U.S. Pat. No. 4,452,880 (Seino et al) describes silicate-containing developers wherein the SiO.sub.2 to alkali metal oxide ratios are much higher, that is between 1.6 and 2.0, and the % SiO.sub.2 concentration is from about 2 to about 9%. These developers have relatively low activity, resulting in slow or incomplete development within the time necessary for practical commercial use. Thus, higher amounts of silicate must be included. Such higher amounts can be disadvantageous due to cost, residue on the plates and the potential for clogging processing equipment.
Copending and commonly assigned U.S. Ser. No. 08/729,471, filed by Miller and Felker on Oct. 11, 1996, and U.S. Ser. No. 08/729,472, filed by Miller, Stuber and Felker on Oct. 11, 1996, describe alkaline developer compositions that overcome the problems noted above and provide a significant advance in the art. Both applications describe compositions containing specific amounts of silicates, and preferred compositions also include at least 6% (by weight) of a water-soluble or water-dispersible thickener, such as glycerine. However, there is a need to further improve such compositions so they provide optimal performance with infrared radiation sensitive ("thermal") lithographic printing plates. Particularly, it is desired to provide greater processing latitude, and to better protect imaged areas of the plates during alkaline processing.